The present invention relates to the field of high frequency converters and, more particularly, to a high frequency push-pull AC/AC converter with input power factor correction.
High frequency converters are known and have many useful applications in electronic technology. One particularly attractive application thereof is for the supply of regulated electric energy to electric discharge lamps. One of the desirable features of a high frequency converter is that it should have a high power factor as seen from the 60 Hz (50 Hz) AC power lines.
One example of a conventional high-frequency AC/AC converter system with a corrected input power factor is shown in FIG. 1, where a boost converter is provided for power factor correction and a push-pull inverter is used for high-frequency DC/AC power conversion. An electro-magnetic interference filter (EMI filter) is coupled to a pair of input terminals which supply, for example, a 60 Hz sinusoidal AC line frequency of a nominal voltage of 115 volts. The EMI filter is used to filter out the high frequency noise generated by the high frequency operation of the boost converter and the push-pull inverter. The AC line voltage is rectified by a full bridge diode rectifier circuit.
In FIG. 1, a boost converter 1 which provides power factor correction is coupled to the output terminals of the bridge rectifier circuit. The boost converter in its elemental form consists of an inductor L and a diode D connected in series circuit between a first input terminal and a first output terminal of the boost converter. The second input terminal and the second output terminal of the boost converter are connected in common. An energy storage capacitor C.sub.e is connected across the first and second output terminals of the boost converter. A switching field effect transistor Q is connected to a junction point between the inductor L and the diode D and to the common line connecting the second input and output terminals. The body diode and inherent capacitance C.sub.ds of the MOSFET Q is shown in parallel therewith. The gate electrode of the MOSFET Q is coupled to the output of a control circuit A.
In the boost converter stage, the current flowing through the inductor L is monitored and is compared in the control circuit A with reference values generated from the rectified voltage. The current waveform is shaped to have the same waveform as and to be in phase with the rectified voltage waveform by controlling the ON duty ratio and/or the switching frequency of the MOSFET switch Q. The output of the boost converter is a DC voltage roughly regulated by the control circuit A. This DC voltage is then inverted into a high frequency AC voltage by the high frequency push-pull DC/AC inverter 2.
The high frequency DC/AC inverter has its input terminals connected directly to the output terminals of the power factor correction boost converter 1. A first input terminal of the high frequency inverter is connected to a common junction point of a pair of primary windings Np1 and Np2 of an output transformer T. A secondary winding Ns of the transformer is coupled to a series circuit consisting of an inductor Lr and a capacitor Cr. The load is connected across the terminals of the capacitor C.sub.r. One end of the primary winding Np1 and one end of the other primary winding Np2 are connected to the common output line via the MOSFET transistor switches Q1 and Q2, respectively. The respective body diodes and inherent capacitances of the MOSFET transistors Q1 and Q2 are also shown in the drawing. The gate electrodes of the MOSFET switching transistors Q1 and Q2 are connected to respective outputs of a control circuit B.
The high frequency power developed in the high frequency push-pull DC/AC inverter 2 is delivered to the load via the transformer T. The control circuit B has an input coupled to the load and is used to feed back the output power so as to control the operation frequency of the MOSFET switches in the high frequency DC/AC inverter so that a regulated output power can be obtained. Since the input power of the system has a low frequency (100 Hz or 120 Hz) component and the output power of the system is a regulated high frequency power, the storage capacitor C.sub.e is required between the boost converter and the push-pull inverter for energy storage in order to balance the input power and the output power.
An important disadvantage of the conventional high frequency converter circuit shown in FIG. 1 is that the voltage stress on the MOSFET switches Q1 and Q2 in the push-pull inverter 2 is high. If the ON duty ratio of the MOSFET switch Q in the boost converter is 50 percent, the voltage across the energy storage capacitor Ce will be twice the amplitude of the AC line voltage. In this case, the voltage stress on the MOSFET switches in the push-pull inverter will be four times the amplitude of the line voltage. The voltage stress can be reduced by employing a smaller duty ratio. However, the voltage across the energy storage capacitor will always be higher than the amplitude of the line voltage due to the boost converter operation. Typically, it is designed to be 1.5 times the amplitude of the line voltage. In that case, the voltage stress on the MOSFET switches Q1 and Q2 in the push-pull inverter will still be three times the amplitude of the line voltage.
It is also known to use a half-bridge inverter instead of the push-pull inverter in order to reduce the voltage stress on the MOSFET switches in the inverter. However, in this case, one of the two MOSFET switches is in the high voltage side of the circuit. A high-side driver is then required to drive the high side MOSFET, resulting in additional cost and complexity of the overall circuit.
In order to simplify the control circuit A, a discontinuous conduction mode (DCM mode) is usually adopted for the boost converter for power factor correction. In the DCM mode, the current flowing through the inductor L is discontinuous. If the ON duty ratio of the MOSFET Q is fixed, the peaks of the current through the inductor L will follow the waveform of the rectified line voltage. Therefore, a high input power factor can be obtained after the high frequency components of the inductor current are filtered out by the EMI filter. In this case, the feedback of the rectified voltage and the monitoring of the inductor current can be eliminated.
The conventional high frequency converters discussed above each include two high frequency power stages and two corresponding control circuits. One stage is for the input power factor correction and the other stage is for the DC/AC power conversion.